The present invention relates generally to cathode ray tubes of the type that include electron lenses for amplifying deflections of their electron beams, and more particularly to an improved box-shaped scan expansion lens for such tubes.
Much work has been done in recent years to produce shorter, large screen oscilloscope CRTs having high deflection sensitivities and good spot characteristics. To obtain the required deflection sensitivity, some form of deflection amplification, also referred to as scan expansion or scan magnification, is required in such tubes. One of the more popular ways to achieve this has been to use a dome-shaped mesh to modify the field between the deflection plates and screen of a CRT, as disclosed, for example, in U.S. Pat. No. Re. 28,223 to Odenthal et al. While capable of producing excellent display characteristics, such meshes intercept a portion of the tube's electron beam. This causes a reduction in beam current, and hence in writing speed, a loss of contrast due to secondary emission from the mesh, and defocusing of the spot.
These limitations of the domed mesh can be overcome by the use of a three-element axially symmetric lens, such as that described by Schackert in IEEE Transactions on Electron Devices, Vol. ED-18, No. 8 (Aug. 1971), or by the use of electrostatic quadrupole lenses, such as described in U.S. Pat. No. 3,496,406 to Deschamps. Because of limitations imposed by its axial symmetry, and because the horizontal and vertical deflection centers are imaged by the lens in different ways, the three-element lens cannot achieve the geometry and linearity characteristics required for a precision oscilloscope display. Quadrupole scan expansion lenses, while capable of producing good display characteristics, require the use of an additional quadrupole, located between the horizontal and vertical deflection plates, to obtain proper focus. This imposes restrictions on deflection plate length, limiting performance.
All of the just-described scan expansion lens systems are designed for use in CRTs having post deflection acceleration (PDA). Thus, in addition to their other drawbacks, none of these systems is suitable for use in monoaccelerator tubes, such as storage CRTs.
A further type of electron lens, not heretofore utilized for deflection amplification, is disclosed in U.S. Pat. No. 2,412,687 to Klemperer. The patent describes several electron lens systems, including one consisting of multiple aligned tubular lens-forming electrodes having oppositely curved adjacent end surfaces. Such lenses are said to be useful for focusing a flat ribbon-shaped beam in a line.
It is therefore a general object of the present invention to provide an improved CRT scan expansion lens that is free from the above-mentioned disadvantages of prior art lenses.
A more specific object of the invention is to provide a scan expansion lens that is adapted for use both in monoaccelerator and PDA cathode ray tubes.
A still more specific object of the invention is to provide a box-shaped scan expansion lens comprised of axially aligned tubular elements of rectangular cross-sectional configuration, with adjacent elements being spaced apart and suitably curved along their opposed end edges to provide curved electron lenses between each pair of elements upon the application of different electrical potentials thereto.
Another object of the present invention is to provide a box-shaped scan expansion lens system that combines high deflection sensitivity and small spot size characteristics suitable for high precision oscilloscope applications.
Still another object of the invention is to provide a CRT scan expansion lens that can be adjusted to minimize display distortions.
Additional objects, features, and advantages of the present invention will become apparent as the following description of preferred embodiments thereof is read in conjunction with the accompanying drawings.